The present invention relates to switching amplifiers and more specifically to techniques which enable switching amplifier architectures which employ noise-shaping feedback techniques to generate output waveforms which more closely resemble their input waveforms than conventional architectures.
Typical digital audio systems generate a one-bit digital signal (single-ended or differential) with some form of modulation, e.g., pulse width modulation (PWM), to a digital amplifier for amplification. The audio information is represented by the timing of transitions in this one-bit signal which is typically in the frequency range of 200 kHz to 1 MHz. This signal is also often enhanced by some form of digital audio processing before amplification. Most such systems employ an a priori approach for mitigating distortion, i.e., the signal processing attempts to mitigate expected distortion from subsequent stages. However, such an approach is largely ineffective with regard to the largest source of distortion in the system, i.e., the power stage. In fact, many of most sophisticated digital audio signal processing solutions available today ignore, and are therefore rendered ineffective by power stage distortion.
One highly successful approach to eliminating or reducing power stage distortion was pioneered by Tripath Technology Inc. of San Jose, Calif. This approach employs a modified, oversampled, sigma-delta modulator architecture which uses continuous-time feedback from the output of the power stage to effectively push the power stage distortion out of the band of interest. Such a noise-shaping approach is described in detail in U.S. Pat. No. 5,777,512, the entire disclosure of which is incorporated herein by reference.
As described in that patent, specific implementations of the modified sigma-delta modulator accept an analog input. Thus, when such modulators are integrated with audio systems which employ a digital format for the source, e.g., virtually all audio systems today, this makes some form of digital-to-analog (D/A) conversion necessary. Thus, despite the significant reduction in the distortion of the output signal, inclusion of this D/A conversion adds to the cost and complexity of the overall system.
In addition, some implementations using the approach described in the above-referenced patent result in output waveforms having characteristics which do not always closely resemble the input waveforms with regard to one or more characteristics, e.g., frequency. While the introduction of such artifacts may be acceptable from some applications, e.g., some audio applications, there are other applications for which such artifacts are undesirable.
It is therefore desirable to mitigate power stage distortion switching amplifier architectures while faithfully reproducing input waveforms at the amplifier output.